The wearable market is exploding and the heart rate monitors and other measuring devices are becoming increasingly important. Many individuals are becoming more conscious of their health and many others are suffering from heart diseases. To aid these individuals and patients a cardiac pacemaker is surgically placed in the body. Pacemakers have a problem of occasional failure or premature discharging of the batteries. In hospitals there are testing devices for the monitoring of heart rate. However, usually these devices are quite bulky. The real problem associated with these essentially stationary testing devices is that they are intrusive and are not adapted to be worn by the patient outside of the hospital. Therefore a number of heart rate sensors were designed and fabricated that was based on different physical principles. Out of these the simplest and reliable sensors are based on strain sensors.
For heart rate measurements a finger-mounted strain sensor for the measurement of small volume of blood changes can be used (Aston R., et al 1991). A strain sensor is a device that converts the changes in the length (Δl) of the sensor or object into change in the resistance (ΔR) in the sensor. Strain (S) is ratio of Δl to length (l):S=Δl/l  (1)Sensitivity of sensor or gauge factor (G) is:G=ΔR/RS  (2)
The strain sensitivities of some sensors that are used in practice, for example, for constantan and silicon are equal to 2.1 and 120, respectively. Cardiac monitor that can be strapped around the wrist (where a radial pulse is normally detected) was developed (Manual B et. al., U.S. Pat. No. 3,742,937). The sensor can detect heart rate and abnormalities in the heart beat rate.
Broadwater et. al (1982) fabricated a digital watch for the measurement of systolic and diastolic blood pressure and heart rate. This watch has a piezoelectric transducer that is held in contact with the wrist adjacent to the radial artery. The electronic circuitry of the watch allows to detect blood pressure pulses and systolic and diastolic pressures.
Pinter and Muehlsteff (2009) have developed a sensor for the combined heart rate and respiration measurement of a patient. The sensor consists of a strain gauge that is assembled on an elastic carrier and two stickers for sticking the sensor on the patient's chest. The movements of the patient's chest due to heart beat and/or respiration lead to a varying strain gauge signals.
Rytky (US 2006/0142654 A1) fabricated a sensor system, a garment and a heart rate monitor. The sensor system comprises at least one flexible film structure consisting of a first insulation layer, at least one electric conductor layer formed on the top of first insulation layer and an electrode area, which is configured to establish an electric contact with the surface of the user's skin and to generate an output signal (electric signals) proportional to a momentary value of the electrocardiogram.
The invention by Sullivan (U.S. Pat. No. 7,689,271) determines the heart rate and respiration rate of a patient through the patient's extremities. Heart rate and respiration rate are determined via an energy spectrum, period gram or histogram using a time series analysis. A patient can stand near the device and lean on it, or stand on a piezoelectric pad. A microcomputer provides calculations to determine heart and respiratory rates using signal processing and time series analysis of data. For heart rate measurements, a finger-mounted or strapped about the wrist strain sensor (which measures small changes in the volume of blood) can be used. The strain sensor is usually connected in one of the arms of Wheatstone bridge. Sensitivity, reliability, cost and size of the strain sensors depends on materials, technology of fabrication and sensor circuit. This circuit connects the sensor or sensors to electronic circuit that filters, amplifies counts (heart pulse rates) and displays them on the screen. However, there is a need for a better composite that is cost effective, accurate and sensitive.